Run SFC /Scannow on external drives

Martin Brinkmann
May 21, 2017

The command sfc /scannow is a useful Windows command that runs a system file check to make sure essential files are not damaged or missing.

This can be mighty useful in many situations, for instance when Windows features don't work anymore, after an unexpected shutdown of the system, or after a virus attack.

The default command runs the file verification check on the active partition. What if you want to run a check on an external drive?

I ran into an issue today for instance where my main PC would not boot anymore. Windows ran an endless "trying to repair" loop but would not budge at all. I made the decision to connect the drive that Windows was installed on to another computer system, to run checks on it. One of those checks was the system file check using sfc.exe.

Run SFC /Scannow on external drives

sfc scannow external drive

You may run the sfc /scannow command on external drives, or internal drives with another Windows installation.

The process is nearly identical:

  1. Tap on the Windows-key on the keyboard, type cmd.exe, hold down the Ctrl-key and the Shift-key, and hit the Enter-key. This opens an elevated command prompt.

If you are interested in all switches that SFC supports, type SFC /?

The following command needs to be used to run the system file scan on another directory that is not the active system directory:

sfc /scannow /offwindir=d:\windows /offbootdir=d:\

Note that you need to replace d:\windows with the correct directory. If the Windows installation is on t:\win, replace the instance of d:\windows in the example above with those.

The Windows tool runs a file system verification scan on the selected Windows directory, and tries to repair any issues that it finds.

Note that I'm not entirely sure how this works if the host system runs a different version of Windows than the system that is checked.

You can run the other sfc commands as well on other Windows directories:

  • /verifyonly -- This is like scannow, but won't touch the files but simply validate them. Useful to verify without modifying any files potentially.
  • /scanfile=file -- This runs the scan on the selected file only, and not the full directory.

Messages that SFC may return

SFC may return the following messages:

  • Windows Resource Protection did not find any integrity violations - No issues were found. This is good usually, unless you try to repair something.
  • Windows Resource Protection could not perform the requested operation -- You won't get this error when you run SFC on an external drive or directory. If you do get the message, run SFC in the Safe Mode environment.
  • Windows Resource Protection found corrupt files and successfully repaired them. Details are included in the CBS.Log %WinDir%\Logs\CBS\CBS.log -- Consult the log for information on what it fixed.
  • Windows Resource Protection found corrupt files but was unable to fix some of them. Details are included in the CBS.Log %WinDir%\Logs\CBS\CBS.log -- Like above, but now you need to consult the log to find out what SFC could not fix.

Closing Words

The file verification check can be useful, but it is not a 100% method of correcting Windows issues. I had to restore an earlier backup for instance, as the sfc command would not find anything wrong with the system files.

Now Read: SFCFix may help if SFC does not fix issues

Run SFC /Scannow on external drives
Article Name
Run SFC /Scannow on external drives
Find out how to run file system verification scans using the sfc tool of Windows on external Windows installations and drives.
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  1. Dave P said on May 4, 2023 at 10:34 am

    This would be a (very) useful post if the fact that scannow NEEDS a bootable copy of the OS to be repaired to do its thing. So, if you’re trying to repair XP SP3, then you’ll need a bootable CD (or usb iso) of that. Quite how this scenario plays out viz. switches when repairing an externally-mounted faulty boot drive is uncertain, and as the need for these media is not mentioned this article is little use. No wonder no-one in the comments is getting it to work…

  2. Alan said on December 29, 2022 at 5:28 pm

    for those getting the error: could not start service etc on the external drive you need to do this:
    worked for me. maybe close cmd and start again first.

  3. Jim P said on November 8, 2022 at 4:57 pm

    I know I’m a little late to the party – but, I have an external Toshiba Canvio that I can see in Device Manager (where it tells me this device is working properly, while telling me its not initialized when I try to populate the volumes), shows up as “disk 2” Unknown and not initialized in Disk Management with no mention of storage space available or used. I’ve gone through and have updated all of the USB drivers (I can’t find one for this Toshiba Canvio though) by uninstalling the USB device from device manager and rebooting the machine – nope, nothing changed. I’ve purchased a new micro B cord in case that was the issue (it wasn’t). I’ve tried initializing it – but I get “drive isn’t ready”. This is something I’m trying to help a friend out with and I’ve had some tough drives to crack before but this is driving me crazy.

  4. Jack W said on October 22, 2021 at 3:20 pm

    So many articles talk about scannow on external ssd….but if it is ONLY storage, no boot,etc. Is it needed to fix fiie errors on your storage files? Much thanks Msttin! Lovvvvvve your site!

  5. el jefe said on May 4, 2021 at 8:05 pm

    Another Message SFC may return: Windows Resource Protection could not perform the requested operation.

    Attempting to run on external drive.

    1. Anonymous said on October 7, 2021 at 6:04 pm

      I got the same, which contradicts the Messages section above.

  6. Anonymous said on May 5, 2020 at 2:15 am

    Okay, SFCFix from Major Geeks worked on my Windows 10 machine. Thank you, Gary D.

  7. Bob said on February 11, 2020 at 1:43 am

    I run Win/10 and have a drive C,D,E & F;the latter two being external HDs and C&D being internal.Tried doing thru CP and it says”Windows Resource Protection could not start the repair service.”.So I did as you say in safe mode and still no go.

    1. Harry said on May 5, 2020 at 2:06 am

      Yeah, same issue for me using Widows 10. Bob.

  8. Gary D said on May 22, 2017 at 1:48 am

    If sfc doesn’t fix the problems, I use sfcfix v3.0 which can be downloaded from Major Geeks.

    Refer to Martin’s blog from 6/11/15. You can find the blog easily by typing “sfcfix” (without the quotation marks) into the search box above right.

  9. ddk said on May 21, 2017 at 10:58 pm

    “If the Windows installation is on t:\win”

    From the article above. That looks like the letter “t”. I only have 2 Windows installations on drive C and D according to explorer. Never have put anything on T is there a reason anyone would want to?

    1. jasray said on May 22, 2017 at 12:06 am

      Oh yeah, for sure. I mean, “t” is only being used as an example–could have put “z” or some other letter. Desginating a drive as “t” really comes in handy when “testing” a version of an Windows. Kind of jogs the memory “t” for testing.

      sfc /scannow /offwindir=t:\windows /offbootdir=t:\

      I like “z” alot; in fact, I don’t have a drive with an assigned letter C, D, E, etc.

      sfc /scannow /offwindir=z:\windows /offbootdir=z:\

      Old school learned to go backwards with drive letter assignments for various purposes. It makes for some great reading/research.

      1. Danie said on December 20, 2017 at 1:35 pm

        I just keep getting
        ‘… could not start the repair service’

        What am I meant to do?

      2. Harry said on May 5, 2020 at 2:04 am

        I got the same error when I used sfc /scannow /offwindir=d:\windows /offbootdir=d:\ to scab D: (yeah, I have a D: drive for data, in fact):

        Windows Resource Protection could not start the repair service.

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